When a natural gas or oil deposit reaches the end of its normal lifetime, the major part of the hydrocarbons (up to two thirds) remains in the ground, since they are too difficult or too expensive to extract. It is estimated that recovery of just 1% more throughout the world would be equivalent to 20-30 billion extra barrels of oil.
Techniques of assisted hydrocarbon (oil and natural gas) recovery by chemical means involve injecting an aqueous phase into the underground reservoir via injecting wells, which are situated at a distance from the producing wells. The injected aqueous phase maintains the pressure in the reservoir and displaces the hydrocarbons toward the production wells. The aim is also to increase the fluidity of the oil to be recovered, or to reduce the permeability of certain subsoil strata whose characteristics are detrimental to effective scavenging of the reservoir.
The efficacy of these methods is limited by the difference in viscosity between oil and water, which causes the water to seek to pass directly from the injection well to the production well.
The aqueous phase for injection into a well is usually admixed with chemical products whose role is to increase its viscosity, allowing more effective scavenging of the deposit as a whole. The increase in viscosity is manifested in a reduction in the mobility ratio between the aqueous phase and the hydrocarbon phase.
The skilled person is aware that an increase in the viscosity of an aqueous solution with very low levels of additive can be obtained through the use of water-soluble polymers which have a very high molar mass and/or possess monomer units which. are charged (in particular by acid groups), or through the use of hydrophilic biopolymers which give rigid structures.
Charged water-soluble polymers (such as HPMAs or high molecular weight polyacrylamides, which are acrylamides copolymerized with an ionic monomer) have a viscosifying character by substantially increasing the radius of gyration of the molecule, by virtue of the repulsive interactions of the charges present in the molecule. The presence of salts or a change in pH in the medium may “mask” these charges and suppress these interactions, and thereby suppress the viscosifying effect.
Hydrophilic biopolymers such as scleroglucan are very effective rheology modifiers, but have a great sensitivity to bacterial degradation. These molecules are “cut” by certain microorganisms, and thus loose any viscosifying and shear-thinning properties.
Other polymeric compounds have been used as rheology modifiers, examples being hydrophobically associative polymers (HAPs), which have a hydrophilic backbone and, along the chains, comprise small amounts of hydrophobic monomers which are able to combine in water in the form of hydrophobic nanodomaines. These nanodomaines act as points of temporary crosslinking and endow the HAPs with a marked shear-thinning character.